I am currently doing a full installation and there are two sub consumer units to be fitted. The main consumer unit will be a 17th edition with the subs being fed from MCB's from the main unit. Question is, would it be best to fit the sub units with rcd's or will the rcd's one the main unit suffice. If I need rcd's on the subs, I intend to fit delayed rcd's and am I right in saying that the delays are fitted to the main consumer unit and the standard ones to the subs. Cannot appear to find anything in the regs that tell me, but then again it's probably there and I just cannot find it!
It's usual to have the time delayed rcd at the origin and standard ones at the local c/unit. Why are you thinking you need rcd at the origin, is it due to cable installation method or perhaps it's a TT installation?
Its slightly more complicated then simply deciding if you need TD RCD's or not.. The deciding factor will be how are the sub main feeds being installed ? But if you do need to fit RCD's you will need 100mA TD at the origin and 30mA in the sub main boards..
You can't get a 30mA time delayed, for very good reason! The submain cables may need 30mA protection, and if so, there is then no point in 30mA RCDs in the sub board. If you can install your submains in such a manner that RCD protection is not required, you can use "17th" type boards everywhere, omitting the RCD protection at the main board for the two submain circuits. That would be the best method.
"RCD or no RCD" ............ That is the question .............. Shakespark 2010 ...... Qoutation taken from AMPlet
Thanks for your replies, sorry I forgot to mention that this is a TT installation, the subs are going to be in a boiler house/mini workshop, fed by buried SWA, the other is feeding an adjoining building for sockets and lights fed by t&e with dot and dab boards covering. From what I can gather from your comments I feel that the RCDs on the main board would provide sufficient protection for the installation, I think I was thinking that as I am installing another CSU, that The RCD question popped in there! Unless anyone can tell me something different?
As it's SWA, that requires no RCD protection, so place your submains on MCB's in non protected ways in the main CU. Then fit CU's with RCD incomer or RCBO's at the submained locations (or a dual split if there are many circuits). RCD protecting the origin of a submain, especially if via an RCD doing other circuits, should be avoided. You are trying to split the installation into sections protected by different RCD's to comply with the reg which requires the installation is split to prevent danger and inconvenience. As it's TT, are you heading the whole installation with a time delayed RCD (100mA for example?)
Wow, Lectrician, I really need to get my head back into the books! What you say makes perfect sense, the subs are only small installations (2/3 circuits) so will not bother with split CU's, will do what you suggest and as a TT system will put a time delayed RCD to protect the whole install. Thank very much.
You still need a dual split at origin. This provides your 30mA supplementary protection for sockets, buried cables and bathrooms. It's a dual split to keep circuits on in the event of one trip operating. The time day 100mA (or larger) is required at origin on a TT due to increased loop impedance. This allows circuits the submarine to be unprotected by the 30mA, the RCD being at the end of the submain, where it is more convenient. You still need 30mA protection were required even with the time delayed RCD.
As the installation is TT, it may well require RCD protection for the SWA. The maximum disconnection time for Distribution circuits in a TT installation is 1s. I don't know if it is possible to obtain a time delayed RCD that can be set to 1s.
Why would you want to "set it to 1s" ? There is no need. A 100mA S RCD is all that is needed to protect the submain feed..
Because if you dont fit a 100mA S RCD at the mains you wont achieve the discrimination required from the 30mA RCD in the submain board.
S type RCDs are time delayed. The maximum disconnection time allowed by BS7671 for distribution circuits in TT installations is 1s. An 'S' type RCD should operate within 200ms plus whatever time delay is selected. As such an 'S' type RCD set to 1s time delay will not satisfy the requirements of BS7671. It would have to be set to 800ms.
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